1. Field of the Invention
The present invention relates to interference reduction in digital communication systems in general and more specifically to inter-channel interference reduction in mobile, wireless OFDM systems.
2. Description of the Related Art
Advanced multimedia services continue to drive requirements for increasing data rates and higher performance in wireless systems. High performance communication systems such as those specified by the European terrestrial digital video broadcasting (DVB-T) standard and the Japanese integrated services digital broadcasting terrestrial standard (ISDB-T) may use communication methods based on Orthogonal Frequency Division Multiplexing (OFDM). The performance of an OFDM receiver may be sensitive to inter-channel interference between different sub-carrier frequencies in a time-varying environment, such as for a mobile, wireless device.
A received OFDM symbol in an OFDM system may consist of both data and pilot synchronization information transmitted on multiple sub-carriers multiplexed together and spanning multiple sample periods. Modulation and demodulation in an OFDM system may use an inverse fast Fourier transform (IFFT) at the transmitter and an FFT at the receiver respectively. At the transmitter, a cyclic prefix of a section of the IFFT output for each OFDM symbol may be appended to the beginning of the OFDM symbol as a guard interval (GI) to provide some protection from inter-symbol interference (ISI). The length of the OFDM symbol before adding the guard interval may be known as the useful symbol period duration. At the receiver, the cyclic prefix (guard interval) may be removed prior to the FFT demodulation by positioning appropriately an FFT window, whose size may be equal to the useful symbol period duration, along a received sample sequence. The FFT demodulation may transform the window of received time domain samples, in the received sample sequence, to a frequency domain (OFDM) symbol. While the addition of the cyclic prefix guards against ISI, it may not compensate for inter-channel interference (ICI) that may occur between sub-carriers in an OFDM symbol when the communication channel varies in time during the OFDM symbol.
Prior art techniques for ICI mitigation may estimate an amount of ICI in a receive OFDM symbol by estimating the communication channel's frequency domain transfer characteristic, the rate of change of the transfer characteristic and the transmitted OFDM symbol. An ICI estimate may then be subtracted from the receive OFDM symbol, thereby resulting in an ICI-reduced receive OFDM symbol. The performance of the ICI mitigation method may depend on the quality of these estimates, especially in a rapidly time-varying mobile environment. Some prior art methods to provide accurate estimates may be too complex and costly to implement, while others may be too simple to provide excellent performance. Thus, there exists a need for a readily implementable ICI reduction technique that estimates the ICI accurately and exhibits high performance in a mobile environment.